Power factor tap for high voltage bushing



May 17, 1960 E. F. cRoNlN ErAL PowER FACTOR TAP FORHIGH VOLTAGE BusHING Filed May s1, 195s .mM 5 w Ww w fw r/,FUW f 6 T3?? f m a w www Wj by United States Patent O POWER FACTOR TAP FOR HIGH VOLTAGE BUSHIN G Edward F. Cronin, Lenox and William A Wooldrid Q ge Pittsfield, Mass., assigntirs to General Electric Com pany, a corporation of New York Application May 31, 1956, serial No. 588,404 4 claims. (c1. 339-36) This invention relates to electrical insulating bushings, and more in particular, to an improved low voltage outlet bushing adapted for application as a power factor tap for high voltage electrical insulating bushings.

High voltage electrical insulating bushings are commonly employed on such apparatus as high voltage stationary electrical induction apparatus for the purpose of providing means for carrying high voltage leads through the walls of the apparatus. Such bushings are generally comprised of a central conductor, which may be hollow, extending through the high voltage bushing from a terminal assembly at one end thereof. A ceramic insulating shell surrounds the portion of the conductor adjacent the terminal assembly, and extends from the terminal assembly to a centrally located metallic mounting flange. Surrounding at least the central portion of the central conductor and inside of the insulating shell is an insulating core member which may also comprise the external surface of the other end of the bushing.

In bushings of the class adapted for use at 69 kilovolts and below, the insulating core is generally comprised of a resin impregnated laminar material. A plurality of conducting equalizers may be interwound with the insulating core if the bushing is of the capacitor type. The chamber in the bush-ing between the insulating core and the insulating shell is generally filled with a dielectric iluid for improved dielectric strength.

In this type of bushing, a grounding sleeve may be provided surrounding the core and extending from the mounting flange a predetermined distance toward the other end of the bushing.

It is occasionally desired to provide means fo-r evaluating the quality of a high voltage bushing, and in order to achieve this, several means have previously been employed. In the previously described type of bushing, means are frequently employed for isolating the ground sleeve from the metallic mounting flange only during the time that quality tests are being made. In order to provide such isolation, a metallic ring is generally providedbetween the metallic mounting flange and the insulating shell, the ring being electrically connected to the grounding sleeve and insulated from the mounting flange. During normal operation of the bushing, means are provided for electrically connecting the ring to the mounting ilange in order that the ground sleeve will have the same potential as the mounting flange. rl`he connection means between the mo-unting ange and the ring generally comprised a braided conductor o-r a connecting` strap extending between exposed terminals.

The provision of the metallic ring has lbeen found to have certain disadvantages. For example, the axial length of the bush-ing was thereby increased, the mounting studs for the bushing generally extended through the ring and therefore had to be yinsulated from the ring, and the danger of contamination of the insulating gaskets between the ring and the mounting ange required that all such insulating members be cleaned land dried prior to the making of equality test in order to obtain accurate re- ICC sults. Since the quality tests were made of the insulation between the ground sleeve and the central conductor, the quallty tests necessarily included insulating eects of the external surfaces of the bushing. Since the connection between the mounting flange and ring was exposed, quality tests were occasionally erroneously made between the mounting flange and the central conductor.

Bushings adapted for use above 69 kilovolts, requiring better electrical insulation, generally have a ceramic shell dening a dielectric Huid filled chamber at the lower end of the bushing instead of solid insulation. The core of this type of bushing is usually comprised of a plurality of layers of dielectric lluid permeable material, such as paper, interspersed with a plurality of conducting equalizers. On this type of bushing, a projecting boss is frequently employed on the mounting flange, a low voltage electrical insulating bushing is provided in an aperture in the boss, and the central conductor of the low voltage bushing extended from the outermost equalizer of the high voltage bushing core to a terminal end outside of the projecting boss. A metallic cap threaded over the end of the boss served to electrically ground the central conductor of the low voltage bushing during normal usage off the high voltage bushing.

The use of this type of power factor tap had not generally been extended to the lower voltage rated high voltage bushings for several reasons. While this type of power factor tap has the advantage that there is no danger of contamination of insulating surfaces, and also the advantage that there is no danger of erroneous measurement due to mistake of other projections for the power factor tap, previously employed structures for such taps disrupted the electrical ground plane due to their necessary size so that the projecting boss was the critical point on the bushing as far as tendency to spark over, the limitation on the size of the tap being controlled in a large measure by the grounding cover tap of the low voltage bushing which increased the radial distance of the projection with respect to the high voltage bushing as well as the axial dimension of the power factor tap. Thus, in order that the central conductor of the low voltage bushing extend a suicient distance beyond the boss on the mounting ilange for convenient connection of test leads thereto, it was necessary that the grounded cover cap of the low voltage bushing extend even beyond the central conductor of the low voltage bushing. As a result of the necessary dimensions to apply such low voltage power factor tap bushings to the lower rated high voltage bushings, it would have been necessary to redesign the high voltage bushing shell and thereby increase its cost in order to overcome the eiect of the increased external electrical stress on the bushing.

It is therefore an object of this invention to provide an improved low voltage electrical insulating bushing.

Another object is to provide a low voltage electrical insulalting bushing having a minimum axial extension.

Still another object of this invention is to provide a power factor tap for a high voltage capacitor type electrical insulating bushing, the power factor tap having a minimum effect upon the electrical ground plane of the high voltage bushing.

A further object is to provide an improved power factor tap for high voltage capacitor type electrical insulating bushings having the advantages of minimum necessary axial extension of the high voltage bushing, minimum danger of contamination of insulating surfaces which aiect the test results, minimum radial extension of the tap with respect to the axis of the high voltage bushing without reducing the convenience of connecting test leads to the power factor tap, and minimum cost.

Briefly stated, this invention provides a low voltage electrical insulating bushing especially adapted as a power 'factor tap fora high voltage electrical insulating bushing of the capacitor type. The low voltage bushing is comprised of an expansible central conductor slidably mounted in an insulating sleeve in an aperture in the mounting flange of the high voltage bushing. The central conductor has a contact on one end adapted to contact the radially outermost high voltage bushing equalizer. The other end of the central conductor is normally held within the aperture by a removable plug. Upon removal of the plug, resilient means in the central conductor forces the other end of the central conductor a predetermined distance beyond the end of the aperture in the mounting flange, thereby providing a conveniently accessible terminal for power factor testing. Means, such as flange means on the central conductor and the insulating sleeve limit the maximum extension of the expansible central conductor beyond the aperture.

While the specification concludes with claims particularly pointing out and distinctly lclaiming the subject matter which we regard as our invention, it is believed that the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a cross sectional view of the central portion of a high voltage bushing and illustrating the power factor tap of this invention in the normal operating position of the high voltage bushing,

Fig. 2 is an expanded cross sectional view of the power factor tap of Fig. 1 in expanded or testing position.

Referring now to the drawing, and more in particular to Fig. l therein is illustrated a portion of a high voltage bushing 10 having a central conductor 11 extending axially therethrough. The central conductor 11 is surrounded by a core 12 of insulating material, and one or more electrical Stress equalizers 13 are embedded within the core 12. The lower portion of the core is surrounded by a grounding sleeve 14. The central portion of the core 12 is surrounded by an annular metallic mounting flange 14. An insulating shell 15 of ceramic material surrounds the upper portion of the core 12 and central for at least a portion of its length, and an insulating r sleeve 22 having at least a partially threaded external surface is provided in the aperture 21.

A tubular metallic member 23 (Figs. 1 and 2) having an open end 24 and a closed end 25 is mounted for axial movement within the insulating sleeve 22. A contact 26 adapted to contact the equalizer 13 is axially arranged with respect to the tubular member 25, and an elongated portion 27 of the contact 26 extends axially into the open end of the tubular member 23. A resilient member such as helical spring 28 extends between the tubular member 23 and the contact 26 and acts to bias the tubular member 23 away from the equalizer 13. The tubular member 23 is provided with an outwardly extending flange 29 on its open end 24, and an inwardly extending flange 30 is provided on the insulating member 22, the flanges 29 and 30 being adapted tto engage each other to limit the maximum extension of the tubular member 23 from the aperture 21.

Referring now to Fig. 1, a threaded metallic plug 31 is provided in the end of the aperture 21, the plug 31 being arranged to provide an electrical conducting path between the closed end 25 of tubular member 23 and the metallic mounting flange 14. The plug 31 also serves to hold the tubular member 23 a predetermined distance within the aperture 21 against the expanding force on 4 the spring 2S. Thus removal of the plug ungrounds the high voltage bushing equalizer 13 and permits the tube 23 to extend beyond the aperture 21 to facilitate the connection of the equalizer to test apparatus.

As may be seen in Fig. l, the plug 31 is of the recessed type, so that when inserted in the aperture 21, a minimum projection of the plug 31 extends beyond the flange projection 20. With this arrangement, the maximum axial extension of the plug 31 or the projection 20 in the normal operating condition of the bushing 10 is less than or may be conveniently less than the radial extension of the skirts of "the insulator 15. Since such extension of the power factor tap in normal operating condition of the bushing 12 has been kept to a minimum, no redesign of the insulating shell 15 is necessary in order to compensate for variation of the ground plane caused by the power factor tap.

When the plug 31 is removed from the aperture 21, as shown in Fig. 2, the tubular member 23 is forced radially outwardly by the spring 28 so that the closed end 25 of the tubular member 23 extends 'a predetermined distance beyond the aperture 21 of the projection 20. This provides a readily accessible connection to the high voltage bushing equalizer for the purpose of testing the quality of the high voltage bushing. Since the tubular member 23 extends beyond the aperture 21, any conventional type of connection means may be employed to clip on tto the tubular member 23, and no special connecting means are required. The maximum extension of the tubular member 23 beyond the aperture 21 is limited by the engaging of the flange 29 of the tubular member 23 with the flange 30 of the insulating sleeve 22.

By employing an expansible central conductor, comprised of tubular member 23, contact 26 with the elongated portion 27, and spring 28, the power factor tap of this invention maintains a positive connection with the equalizer 13, while providing a minimum extension of the power factor tap assembly when the high voltage bushing 10 is in normal use.

While the previously disclosed invention is primarily adapted as a low voltage electrical insulating bushing for power factor tap applications on high voltage bushings, it is obvious that the low voltage bushing may advantageously be employed for other applications, and therefore we do not intend to thus limit the scope of this invention.

It will be understood, of course, that, while the form of the invention herein shown and described constitutes a preferred embodiment of the invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications thereof. It will also be understood that the words employed are words of description rather than of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the truc spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. Power factor tap means for a high voltage electrical insulating bushing of the type having a central conductor, an insulating core surrounding said conductor and having at least one equalizer, and a central metallic mounting flange surrounding said core, comprising a portion of Said mounting flange which defines a threaded aperture extending radially therethrough so as to expose said equalizer, an insulating sleeve in said aperture, tubular metallic terminal means mounted for axial movement in said sleeve and having an open end facing said equalizer and a closed end away from said equalizer, Contact means contacting said equalizer and having an elongated portion extending into said open end, resilient conducting means axially extending between said contact means and said tubular member and biasing said tubular member axially away from said equalizer, flange means on said tubular member and sleeve limiting the extension of vsaid closed end a predetermined distance out of said aperture, and removable plug means insertable in said aperture to hold said closed end within said aperture.

2. Power factor tap means for a high voltage electrical insulating bushing of the type having a central conductor, an insulating core surrounding the central conductor and including at least one equalizer, and a central metallic mounting flange surrounding the core, said tap means comprising a portion of said mounting flange dening an aperture extending radially therethrough, an insulating sleeve rigidly held in said aperture, an expansible conductor extending through said insulating sleeve comprising a contact engaging said equalizer, a terminal member extending radially outwardly through said aperture, and conducting spring means extending between said contact and terminal member, means limiting the outward radial movement of said terminal member, and remov able metallic plug means threaded in said aperture to hold said terminal member within said aperture and electrically connect said terminal member to said mounting flange.

3. Power factor tap means for a high voltage electrical insulating bushing of the type having a central conductor, an insulating core surrounding the central conductor and including at least one equalizer, and a central metallic mounting ilange surrounding the core, said tap means comprising a portion of said mounting flange defining a threaded aperture extending radially therethrough, an insulating sleeve threaded in said aperture, an expansible conductor extending through said sleeve and having a contact engaging said equalizer, a tubular terminal having a closed end extending radially away from said equalizer, and a conducting helical spring extending between said terminal and contact flange means on said sleeve and terminal limiting the outward radial 'movement of said terminal, said spring normally 4holding the closed end of said terminal radially outwardly of said aperture, and removable metallic plug means threaded in said aperture for holding said terminal within said aperture and electrically connecting said terminal to said mounting flange.

4. A high voltage electrical insulating bushing having a central conductor, an insulating core surrounding said conductor and having at least one equalizer, a central metallic mounting ilange surrounding said core, a power factor tap for said bushing comprising a portion of said mounting ange which defines an aperture extending radially therethrough so as to expose said equalizer, an insulating sleeve in said aperture, and expansible conducting means within said sleeve axially expandable with respect to said sleeve and radially of said flange, having one end resiliently contacting said equalizer, the other end of said conducting means having a normal restrained position within said aperture and an expanded testing position extending from said aperture.

References Cited in the le of this patent UNITED STATES PATENTS 1,634,404 Douglas July 5, 1927 2,403,979 Hill et al. July 16, 1946 2,724,026 Johnson Nov. 15, 1955 2,724,096 Klostermann Nov. 15, 1955 2,757,351 Klostermann July 3l, 1956 FOREIGN PATENTS 415,491 Great Britain Aug. 30, 1934 452,238 France Mar. 3, 1913 

